1. Field of the Invention
The present invention relates to a method for increasing the rising time for a luminous flux used with a lighting circuit for lighting a discharge lamp that contains either no mercury or only a small amount of mercury as a luminescent material.
2. Description of the Related Art
A related art discharge lamp lighting circuit configuration includes a direct-current power source circuit, a DC-AC converter and a starting circuit (i.e., a starter). With this related art configuration, the discharge lamp lighting circuit, while in a steady state, supplies a rated power to a discharge lamp.
To quickly raise the luminous flux of the discharge lamp, during a transition period immediately following the lighting of the discharge lamp, power exceeding the rated power is supplied to the discharge lamp to accelerate the emission of light (see JP-A-9-330795, for example).
For a related art circuit for lighting a discharge lamp containing mercury, during a transition period extending from immediately following the lighting of the discharge lamp until it is shifted to the steady state, a lamp current (or power to be supplied) corresponding to a lamp voltage is regulated, i.e., a control process is performed based on a so-called control line.
When related art discharge lamps are employed as the light source for a vehicle, safety requirements dictate that an adequate startup procedure be provided, so that the luminous flux rises to the steady value as quickly as possible.
However, when a discharge lamp that contains no mercury, or only a small amount of mercury, is to be lighted by using the related art process for lighting a discharge lamp containing mercury, various problems occur. For example, but not by way of limitation, some of these problems are discussed in greater detail below.
FIG. 7 is a schematic graph, showing a time-transient change in a luminous flux. The horizontal axis represents a time “t” while the vertical axis represents a luminous flux “L”.
A graph curve ga in FIG. 7 represents a change in the luminous flux when a discharge lamp containing mercury is lighted using a lighting circuit, while graph curves gb and gc represent changes in the luminous flux that occur when a discharge lamp containing no mercury is lighted using the same related art lighting circuit. The graph curve gb shows an overshoot, while the graph curve gc shows an undershoot. In either case with non- or low-mercury light, deterioration of the rising characteristic of the luminous flux occurs.
The reasons for the foregoing problems are briefly explained below.
(1) When a specific constant power is supplied to a discharge lamp that contains mercury, the lamp voltage begins to increase immediately following the lighting of the discharge lamp. After a short delay, the luminous flux rises. However, when power is supplied to a discharge lamp that does not contain mercury, a constant relationship between the increase in the lamp voltage and the rise in the luminous flux is not established following the lighting of the discharge lamp.
For a discharge lamp that contains mercury and to which power is supplied in accordance with the lamp voltage, a rise in the lamp voltage that precedes a rise in the luminous flux is observed. However, for a discharge lamp that does not contain mercury, such a power supply control does not always work.
(2) When the related art power supply based on the control line is performed for a discharge lamp that does not contain mercury, and the timing for starting the reduction in the supplied power is shifted away from the rising point of the luminous flux, this shift causes the overshoot or the undershoot described above.
For a discharge lamp that does not contain mercury as a luminescent material, there is no material for emitting light immediately after the discharge lamp is lighted. Therefore, as light emission is started from metal iodide, the light flux rises sharply, and the rising point varies instead of being constant relative to the period that has elapsed since the lighting process began and the lamp voltage.
For example, referring to a graph line gd showing a lamp voltage (VL)—power (P) characteristic in FIG. 8, assume that a discharge lamp A, for which a point PA is regarded as the rising point for the luminous flux, is compared with a discharge lamp B, for which a point PB whereat a lamp voltage is higher, is regarded as the rising point of the luminous flux (a point PO0 represents a preferable reference point as the rising point of the luminous flux). Since after the point PA the maximum power is still being supplied to the discharge lamp A, the overshoot occurs, while before the point PB, as the power supplied for the discharge lamp B is gradually being reduced, the undershoot occurs.
(3) Since the initially supplied power must be increased for a discharge lamp that does not contain mercury, and the lamp voltage in the steady state is low, only a small effective range is obtained for the emission acceleration control based on the control line. Thus, the affect of the variance in the lamp voltage supplied to the luminous flux is greater than when a discharge lamp that contains mercury is employed.
FIG. 9 is a graph showing an example lamp voltage—lamp current characteristic, while the horizontal axis represents a lamp voltage “VL” and the vertical axis represents a lamp current “IL”. A power control line “C1” is a control line related to a discharge lamp that contains mercury, and a power control line “C2” is a control line related to a discharge lamp that does not contain mercury. As is apparent from the comparison of the inclinations (dIL/dVL) of the control lines during a transition period, the inclination of the power control line C2 is greater, so that the effective range for the emission acceleration control (see double-headed arrow “R”) is narrow. Therefore, when, for two discharge lamps for which the lamp voltage changes differ, the reduction in the supplied power, according to the control lines, is started at the same lamp voltage level, the time-transient changes in the luminous fluxes greatly different.
For example, even when the lamp voltages of the two discharge lamps A and B reach the same level, for the discharge lamp A, this time immediately follows the rise in the lamp voltage, and the reduction in the power supplied is started. On the other hand, for the other discharge lamp B, this is the time that has elapsed since the rise in the lamp voltage. Accordingly, excessive power is supplied to the discharge lamp B, so that the overshoot occurs (or, when the power supplied is controlled (power reduction) by using, as a reference, the change in the lamp voltage of the discharge lamp B, the undershoot occurs for the luminous flux of the discharge lamp A).